Such a data transmission is already known in the art, e.g. from the published European application EP-A2-0359072. Therein a data transmission system is described which simultaneously codes a main bitstream of high bitrate and an auxiliary bitstream of lower bitrate without increasing the transmission rate of the coded symbols above that high bitrate. This is done by switching between two coding laws as explained hereafter.
In the known system the main bitstream is normally coded according to a first coding law, but a second coding law is selected when a specific bit pattern is detected in the main bitstream, e.g. a string of 8 zeroes. The duration of this change in coding law is determined by the transmitter according to the binary value of the next bit to be transmitted at the auxiliary bitstream. A receiver reconstructs the main and the auxiliary bitstream by detecting this change in coding law and by determining its duration.
In so doing the rate at which the auxiliary bitstream can be coded and transmitted is dependent on the statistical properties of the main bitstream, i.e. on the rate at which strings of 8 zeroes appear. It can therefore not be guaranteed that the auxiliary bitstream can be transmitted at a fixed bitrate relative to the bitrate of the main bitstream. It can also not be guaranteed that the auxiliary bitstream is received with uniform delay. This asynchronous transmission of the auxiliary bitstream results in the need for buffers in the transmitter as well as in the receiver. Whenever a bit of the auxiliary bitstream is available the transmitter has to buffer it together with the following bits until the specific bit pattern occurs in the main bitstream. The receiver on the other hand has to buffer the bits in order to assure a uniform delay for all auxiliary bits.